This invention relates to the preparation of materials that may be used in the electrodes of solid electrolyte fuel cells. In particular, the invention relates to methods of making zirconia-containing fuel cell electrodes that are not prepared from expensive pre-cursor yttria-stabilized-zirconia (YSZ) starting materials.
Solid electrolyte fuel cells include both oxygen-ion conducting solid-oxide fuel cells (SOFCs), and protonic ceramic fuel cells (PCFCs). Each fuel cell includes a pair of electrodes separated by a ceramic electrolyte that permits ions (e.g., oxygen ions, protons, etc.) to migrate between the electrodes as the cell generates electrical current. In solid electrolyte fuel cells, the layer of electrolyte material is often kept thin (e.g., about 25 μm or less) to allow efficient ion migration between the electrodes. Such a thin electrolyte layer made from ion-conducting ceramics is usually too fragile to support itself, and therefore requires an underlying support layer.
In some fuel cell designs, one of the electrodes acts as an electrolyte support in addition to being an electrode. For example, the fuel cell anode may be a self-supporting anode electrode on which the thin electrolyte layer is formed. Not surprisingly, electrodes that act as a support layers use significantly more starting material than non-supporting electrodes.
In the case of anode supported SOFCs, the cells operate at temperatures of about 700° C. to about 1000° C., requiring that the thermal coefficient of expansion (TCE) be closely matched between the electrolyte and the anode to prevent the thin electrolyte layer from fracturing as well as to maintain good adhesion between the layers. One way to match the TCEs is to make a substantial portion the electrolyte and anode out of the same material. A material that has been used successfully in both the electrolyte and self supporting anode of solid electrolyte fuel cells is yttria-stabilized zirconia (YSZ). In the electrolyte, YSZ acts as a good oxygen ion conductor at fuel cell operating temperatures, and in the electrodes YSZ provides a good substrate support for conductive materials that conduct the electrical current. For example, the anode may be made from a mixture of nickel oxide (NiO) homogenously dispersed in YSZ. When the nickel oxide is reduced to nickel metal the material becomes an electrically conductive ceramic-metal composite or “cermet.” The finely dispersed YSZ in the anode also provides the three phase boundary (TPB) between the gas, electrode, and electrolyte.
Unfortunately, conventional methods for making YSZ for solid electrolyte fuel cells are complex and expensive. In one method, powders of monoclinic zirconia (ZrO2) and 8-mol % yttria (Y2O3) are mixed and calcined at high temperature (e.g., about 1700° C.) to form tetragonal and cubic phased zirconia. As the reaction continues, fully yttria-stabilized cubic phased zirconia (i.e., YSZ) results. The reaction is slow, and the conversion of the zirconia into YSZ yttria requires the mixture be kept at high temperature for an extended period of time. As a result, the YSZ is formed as a large grained powder (e.g., grains greater than 1 μm in diameter), which may have to be extensively milled before it can be used in fuel cell electrolytes and electrodes. Other conventional methods of making YSZ, such as spray pyrolysis and co-precipitation, are even more complicated and costly.
The high cost of YSZ produced by conventional methods is an impediment to the widespread adoption of solid electrolyte fuel cells for residential and industrial power needs. This impediment is especially acute for fuel cells with a self-supporting electrode that can make up 90% or more of the total material used in the cell. Thus, the development of less complicated and costly methods of making YSZ for fuel cell electrodes would provide a significant advance in making fuel cells less expensive and more competitive for the generation of electric power.